Each year over 500,000 human joints require replacement as a result of debilitating disease or traumatic injury. Hip and knee joints represent a majority of these cases. To meet this need, a large number of partial and total joint orthopedic implants have been designed and are presently being marketed by various manufacturers. These devices are some of the most remarkable surgical developments of the 20th century because they dramatically restore pain-free mobility to diseased, worn, or traumatized joints and thereby prevent joint dysfunction from limiting the quality and quantity of life. Examples illustrative of the types of orthopedic implants available include but certainly are not remotely limited to U.S. Pat. No. 5,020,063 to Tager, U.S. Pat. No. 5,030,238 to Nieder et al., U.S. Pat. No.5,108,452 to Fallin and U.S. Pat. No. 5,180,394 to Davidson.
Total joint prostheses are secured to the host bone utilizing one of two different techniques, bone ingrowth into specially engineered and manufactured pores on the surface of the implant, or bone cement. Porous ingrowth fixation methods have three limitations. First, implant fixation by porous ingrowth requires a period of post-operative restricted weight bearing. Delays in post-operative ambulation can have adverse clinical consequences on the respiratory system and overall health of older patients. These generally lead to additional costs from prolonged post-operative hospitalization. Second, in the event of infection or excessive bearing surface wear, porous ingrowth total joint implants are difficult to remove (revise) and frequently require fracture of the bone cortex. Finally, there always exists the potential for early failure due to an inability to achieve adequate ingrowth. This means lack of fixation and failure of the prosthesis. When bone ingrowth does not occur, revision joint surgery becomes necessary.
Bone cement is the common name given to polymethyl-methacrylate polymer that in its medical (orthopedic) grade is used as the load-transferring material between a total joint prosthesis and the bone implantation site. Commercially available bone cement is a two-phase material that consists of a liquid methylmethacrylate monomer and a fine pre-polymerized polymethylmethacrylate powder. These components are packaged separately, mixed together in the operating room in a vacuum-mixing chamber and inserted under pressure into the prepared bone cavity before the polymerization reaction is complete. The liquid monomer contains a promoter or accelerator (to initiate the free-radical reaction) and a stabilizer (to prolong shelf-life) and the powder contains an initiator (a catalyst) and a radiopacifier. Radiopaque material is commonly added to bone cement to enable the radiologist to “see” the cement mantle, monitor its integrity and observe the presence of defects.
The name “bone cement” is actually a misnomer because instead of serving as an adhesive, it more accurately serves as a grout or interfacial material between the reamed medullary canal of the proximal femur or tibia for total hip or knee implants, respectively, and the metallic stem of the prosthesis. Bone cement applied to the medullary canal is intended to form a layer (mantle) of uniform thickness between the bone and the implant stem. This cement mantle is intended to mechanically interlock with the pores of the prepared bone and structurally compensate for the inability of the surgical technique to create a cavity in bone that exactly matches the shape of the total joint stem.
Bone cement is the technology of choice for older patients because it virtually guarantees secure, immediate post-operative implant fixation and allows patients to ambulate soon after surgical implantation of the new joint. This avoids respiratory complications, reduces post-operative morbidity and mortality and reduces the duration of hospitalization and rehabilitation. This is particularly important because two-thirds of all hip replacement patients are older than 65 years of age. Total joint prostheses used for cemented fixation are also less expensive than implants used for porous ingrowth fixation and they also have smaller surface areas and thus less likelihood of releasing metal ions into the body. They are also easier to revise in the event of joint bearing surface failure or infection.
Active or overweight total joint patients with cemented implants frequently experience failure of the cement mantle. This occurs in approximately 5% of all such patients about 10 years post-operative. In fact, failure rates as high as 67% after 16 years in patients younger than 45 years old have been documented. Failure of the cement mantle results in loss of fixation, subsidence and motion of the implant in the medullary canal, pain on ambulation, and ultimately, failure of the implant.
Mechanical fatigue fracture of the cement mantle is believed to be one of the chief causes of bone cement failure. Fatigue failure of bone cement is believed to occur in three phases. In the first phase the crack initiates, generally from a flaw in the material's continuity. In the second phase the crack slowly propagates. In the third phase the crack propagates rapidly to failure.
Although recently developed cementing techniques have helped prolong the life of bone cement by eliminating air bubbles in the cement and thereby eliminating this source of stress risers, attempts to augment the mechanical properties of bone cement by the additional of various other materials have generally met with failure. Specifically, stainless steel fibers, polymethylmethacrylate fibers, long macroscopic carbon fibers, polyethylene fibers, aramid fibers and titanium mesh have all been added to bone cement in attempts to bridge bone cement cracks and arrest propagation at stage two. These attempts have been unsuccessful for a variety of reasons including, particularly, the adverse effect such materials have on the mixing of polymethylmethacrylate, the increase in viscosity and the poor fiber/material bonding with the polymethylmethacrylate matrix.
While the prior art teaches that carbon fibers of average length less than 0.1 mm do not provide a desired reinforcing effect in polymethylmethacrylate based resins (note U.S. Pat. No. 4,064,566 to Fletcher et al.), we have now found that carbon nanotubes provide substantially enhanced load bearing mechanical properties to polymethylmethacrylate resins and thereby extend the in vitro service life of polymethylmethacrylate bone cements.
Additionally, carbon nanotube augmented polymethylmethacrylate resins provide improved strength to dental prostheses (e.g. false teeth) to better withstand the forces which are produced in the oral cavity when chewing. Thus, the carbon nanotube augmented polymethylmethacrylate resins provide an excellent material from which one may construct alone or in combination with other compounds dental restorations including but not limited to; dentures, crowns, bridges and other prostheses.